The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Moving Picture Experts Group (MPEG) and Video Coding Experts Group (VCEG) together stepped ahead of the existing MPEG-4 Part 2 and H.263 standard methods to develop a better and more excellent video compression technology. The new standard is called H.264/AVC (Advanced Video Coding) and was released simultaneously as MPEG-4 Part 10 AVC and ITU-T Recommendation H.264.
In such a video compression method, in order to encode an image, each picture is divided into predetermined image processing units, for example, predetermined sized blocks, and the respective blocks are encoded by using inter prediction or intra prediction. In this case, an optimal encoding mode is selected in consideration of a data size and a block distortion degree, and the block is encoded according to the optimal encoding mode selected.
The inter prediction is a method for compressing an image by removing the temporal redundancy between pictures, and a typical example of the inter prediction is a motion estimation coding method. The motion estimation coding method estimates the motion of a current picture in units of a block by using at least one reference picture, and predicts each block based on the motion estimation result.
In order to predict a current block, the motion estimation coding method uses a predetermined evaluation function to search for a block most similar to the current block within a predetermined search range of the reference picture. When similar blocks are searched out, only residual data between the similar blocks in the reference picture and the current block is encoded and transmitted, thereby increasing data compression ratio.
FIG. 1 is a diagram illustrating a method for predicting blocks of a current picture by using a plurality of reference pictures according to related art. Referring to FIG. 1, a plurality of pictures 120, 130 and 140 may be referred to in order to predict blocks 112, 114 and 116 included in a current picture P(n) 110. The picture P(n-1) 120 is located immediately before the current picture P(n) 110 to be the picture temporally closest to the current picture, and a time interval with respect to the current picture P(n) 110 increases toward the picture P(n-2) 130, the picture P(n-3) 140, etc.
Since a plurality of reference pictures are searched for in order to predictively encode the blocks 112, 114 and 116 included in the current picture P(n) 110, reference blocks 122, 132 and 142 used to predict the blocks of the current picture may be from different pictures. FIG. 1 illustrates the case where prediction is performed with reference to the pictures temporally preceding the current picture 110. However, when the current picture 110 is a B-picture (bidirectional predictive picture), not only the pictures temporally preceding the current picture but also the pictures temporally trailing the current picture may be used to predict the current picture 110.
The blocks 112, 114 and 116 of the current picture 110 are predicted to generate respective residual blocks. Then, the residual blocks of the respective blocks 112, 114 and 116, their motion vectors, and reference picture indexes may be encoded to encode the blocks included in the current picture 110. Herein, the reference picture index is information for specifying which of the reference pictures is used for inter prediction.
The conventional hierarchical B-picture based video coding method using hierarchical B-picture is adapted to add a temporal scalability function to a block-based video coding scheme used in the international video standard such as MPEG-1, MPEG-2, MPEG-4 Part 2 Visual, MPEG-4 Part 10 AVC, or ITU-T H.264 in the same way as a block-based MCTF (motion compensated temporal filtering) method, by performing hierarchical predictive encoding with the use of a conventional B-picture for bidirectional motion prediction.
FIG. 2 is a diagram illustrating a general hierarchical B-picture coding structure. As illustrated in FIG. 2, in the case of a hierarchical B-picture coding structure using seven B-pictures, a general hierarchical B-picture coding method sets up potential reference pictures in units of a GOP (group of pictures), and performs reference picture indexing by an inter-picture distance in units of a picture depending on the number of pictures that remain as reference pictures among the potential reference pictures. Herein, the potential reference pictures are I, P and B pictures, and a ‘b’ picture is not used as a reference picture. Thus, the I, P and B pictures among the pictures encoded before a current picture to be encoded are set as reference pictures, and reference picture indexing is performed in the ascending order of distance from the current picture. When four reference pictures are appointed and the current picture is the picture 7, a picture encoding order is 0→8→4→2→1→3→6→5→7→16→12→9→11→14→13→15 . . . . Therefore, when the outstanding picture to encode is the picture 7 with pictures 0, 1, 2, 3, 4, 5, 6 and 8 already encoded, the potential reference pictures are the I picture of picture 0, the P picture of picture 8, and the B pictures of pictures 2, 4 and 6. Thus, reference picture indexing is performed with respect to each reference picture direction according to the distances between the picture 7 and the pictures 0, 2, 4, 6 and 8, which may be represented as Table 1.
TABLE 1Ref_IndexDirection0123L06428L18642
However, since this reference picture indexing method is to find a reference picture by identically indexing identical candidate reference pictures, it may fail to get the very best reference picture to provide the highest coding efficiency for the current block or an arbitrary unit.